Most electrical circuits require a supply voltage for powering the various components of the circuits. Supply voltages themselves are generally maintained within specified limits to assure proper operation of the circuits powered thereby. There are many types of regulator circuits that maintain the supply voltage within prescribed limits. In order to monitor the supply voltage and determine whether it is operating within its limits, a stable reference voltage is used for comparison with the supply voltage. In the event that the supply voltage is too far above the operating range, or too low, an output of the voltage monitor circuit can be used to deactivate the voltage supply itself, or disable the powered circuits so that unreliable circuit operation does not occur.
Voltage monitor circuits are especially useful in processor controlled circuits so that if the supply voltage becomes too low, the processor can be disabled or maintained in a reset condition so that improper processor operation does not occur. In this way, the processor will not process instructions with circuits of the processor operating in an unreliable condition, due to inadequate supply voltages.
There are many other electrical circuits that require a reference voltage in order to compare a stable voltage with an unknown voltage. A reference voltage is a necessary circuit in many analog voltage circuits, such as A/D and D/A converters. Analog comparators in general employ a reference voltage on one input thereof, and the unknown voltage on the other input. The state of the comparator output is an indication of whether the unknown voltage is above or below the known reference voltage.
Circuit designers have typically relied on bandgap circuits to generate precision reference voltages that are stable and highly independent of temperature. The bandgap voltage of a semiconductor junction is utilized in many reference voltage circuits to produce a stable and known voltage. It is well known that the bandgap voltage of a silicon pn junction is about 1.21 volts.
One bandgap reference voltage circuit that is of a typical design is shown in FIG. 1. Here, the voltage reference 10 employs a first diode 12 having a defined pn junction area, and a second diode 14 having a larger area pn junction. There is a resistor 16 that is connected in series with the first diode 12, and a pair of resistors 18 and 20 connected in series with the second diode 14. The resistors 16 and 18 are matched in value. Junction 22 between the first diode 12 and the resistor 16 is coupled to the noninverting input of a feedback amplifier 26. The junction 24 between resistors 18 and 20 is connected to the inverting input of the feedback amplifier 26. The output 28 of the feedback amplifier 26 produces a voltage for driving the equal-value resistors 16 and 18. In order for the feedback amplifier 26 to operate in a state of equilibrium, the voltage at the node 24 must be substantially equal to the voltage of node 22. The values of resistors 16, 18 and 20 are chosen such that when operating at equilibrium, the output voltage of the circuit 10 is substantially equal to a temperature compensated bandgap voltage of the diodes 12 and 14, which is about 1.25 volts. This reference output voltage is very stable and highly independent of temperature variations.
When the feedback amplifier 26 is operating in a state of equilibrium, the junction voltages of the diodes 12 and 14 are somewhat different, due to the difference injunction area. The difference in the junction voltages is reflected across the resistor 20. When the voltages at nodes 22 and 24 are substantially equal, the output 28 of the feedback amplifier 26 is ideally the temperature compensated bandgap voltage of about 1.25 volt.
When utilized to monitor a supply voltage, the reference voltage Vref at the output 28 of the circuit 10 can be coupled to the noninverting input of a comparator 30. The supply voltage (Vdd) is connected to a resistor divider which includes resistors 32 and 34. The node 36 between resistors 32 and 34 is coupled to the inverting input of the comparator 30. The voltage of the node 36 is the threshold voltage which establishes the lower limit of the supply voltage. When the supply voltage is reduced in magnitude, for whatever reason, the threshold voltage at node 36 of the divider will be lowered in an amount proportional to the values of the resistors 32 and 34. If the voltage at node 36 goes below the reference voltage Vref, then the output of the comparator 30 will be driven to a high state. The output of the comparator 30 can be used as a reset signal to a processor to prevent operation thereof when the supply voltage is below a prescribed magnitude. In the event that the supply voltage returns to an acceptable magnitude, the output of the comparator 30 will switch to the other state and allow the processor to resume processing instructions.
While the reference voltage circuit 10 of FIG. 1 is adequate for many applications, there are several disadvantages when employed with processor and other circuits. For example, the use of an amplifier 26 requires additional current from the supply voltage, and the feedback configuration exhibits a second order (or higher) transient behavior, which increases the settling time in order for the circuit output to become stable. Hence, a period of time must elapse before the powered circuits can become operational. This is especially important in processor operations, where additional measures must be taken into account before the processor can start executing instructions in a reliable manner. Another disadvantage to the bandgap reference circuit 10 is that when monitoring a supply voltage, the feedback amplifier 26 cannot often function when the supply voltage is low.
From the foregoing, it can be seen that need exists for a bandgap circuit configuration that is fast reacting, requires less power supply current, and can operate at low supply voltages. A need exists for a voltage monitor circuit that is well adapted for use with reset circuits of processors.